1. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium and a magnetic storage apparatus. The perpendicular magnetic recording medium may be used in an HDD (Hard Disk Drive) or the like.
2. Description of the Related Art
The in-plane magnetic recording system orients an axis of easy magnetization along an in-plane direction of a magnetic recording layer. On the other hand, the perpendicular magnetic recording system orients the axis of easy magnetization along a direction perpendicular to the in-plane direction of the magnetic recording layer. According to the perpendicular magnetic recording system, a demagnetization field becomes small in a vicinity of a magnetization transition region at a boundary between recording bits. Hence, the perpendicular magnetic recording medium becomes electrostatically stable as a recording density becomes higher, to thereby improve thermal stability. The perpendicular magnetic recording system is thus suited for improving the in-plane recording density.
The perpendicular magnetic recording medium includes a soft magnetic underlayer, an underlayer, an intermediate layer, and a perpendicular magnetic recording layer that are successively stacked on a nonmagnetic substrate. In a case in which the soft magnetic underlayer made of a soft magnetic material is provided between the nonmagnetic substrate and the perpendicular magnetic recording layer, the perpendicular magnetic recording medium functions as a so-called double-layered medium that can provide a high recording performance. In this case, the soft magnetic underlayer has a role of circulating a recording magnetic field from a magnetic head, to improve efficiency of the magnetic recording and reproduction.
The underlayer is a dominant element that determines a grain size and the crystal orientation of the intermediate layer and the perpendicular magnetic recording layer that are formed on the underlayer. A material selected for the underlayer is one of main factors determining the recording and reproducing performance of the perpendicular magnetic recording medium. Hence, various underlayer materials have been proposed. For example, Japanese Laid-Open Patent Publication No. 1-263910 proposes using a Ti alloy, and Japanese Laid-Open Patent Publication No. 2003-123239 proposes using an NiFeCr alloy. Materials having an hcp structure, an fcc structure, and an amorphous structure such as Ta have also been proposed for the underlayer. For example, Japanese Laid-Open Patent Publication No. 2010-092525 proposes an underlayer made of an alloy including one of Ni, Cu, Pt, and Pd as a main component thereof, and including one or more added elements selected from Ti, V, Ta, Cr, Mo, and W.
On the other hand, Japanese Laid-Open Patent Publication No. 7-244831 proposes using Ru for the intermediate layer, for example. Tops of columnar crystals of Ru have a dome shape, and crystal grains of the magnetic recording layer or the like can be grown on the dome-shaped tops of the columnar Ru crystals. Japanese Laid-Open Patent Publication No. 2007-272990 proposes promoting an isolation structure of the crystal grains grown on the dome-shaped tops of the columnar Ru crystals, and isolating the crystal grains, to grow magnetic grains in the columnar shape. In addition, Japanese Laid-Open Patent Publication No. 2012-069230 proposes an underlayer having a stacked structure made up of an alloy layer having an fcc structure, including an element having the fcc structure and an element having a bcc structure, and an NiW alloy layer. For example, Japanese Laid-Open Patent Publication No. 2004-227717 proposes using TiV for an underlayer of the magnetic recording medium having the in-plane orientation.
However, there are demands to further increase the recording density of the perpendicular magnetic recording medium.
Various perpendicular magnetic recording media and magnetic storage apparatuses using the perpendicular magnetic recording media have been proposed to satisfy such demands. According to some proposals, the underlayer is made of an NiW alloy, and the crystal grain size of the underlayer is reduced, in order to reduce the crystal grain size and improve the crystal orientation of the intermediate layer and the magnetic recording layer that are formed on the underlayer. These proposals can obtain a good electromagnetic conversion characteristic, and achieve the high recording density. Most of these proposals use a soft magnetic underlayer having an amorphous structure, and provides a microcrystal layer having an fcc structure between the soft magnetic underlayer and the NiW underlayer, in order to achieve microcrystallization of the NiW underlayer. However, the microcrystallization of the NiW underlayer may deteriorate the crystal orientation of the intermediate layer provided above the NiW underlayer.